Being cheerful makes me sorry? Not so! (5)
I believe the answer is:
merry
'being cheerful' is the definition.
(thesaurus)
'me sorry? not so' is the wordplay.
'not' indicates named letters should be taken away.
'sorry' with 'so' removed is 'rry'.
'me'+'rry'='MERRY'
'makes' is the link.
(Other definitions for merry that I've seen before include "Jolly and bright" , "Mistresses Page and Ford of Windsor" , "Lively -- slightly drunk" , "Slightly intoxicated" , "Old King Cole was a ..... old soul" .)